Hydraulic excavators and other construction machines are powered by gasoline, light gas oil, or other fuels, and drive a hydraulic motor, a hydraulic cylinder, or other hydraulic actuators by activating a hydraulic pump via an engine and generating an oil pressure. Hydraulic actuators are compact and lightweight, have high output capacities, and are therefore widely used as the actuators of construction machines.
Meanwhile, as described in Patent Document 1, construction machines using an electric motor to drive electric actuators and save energy by enhancing energy efficiency in comparison with a construction machine that uses only the hydraulic actuators driven by the hydraulic energy of a hydraulic pump, have been proposed in recent years.
Hydraulic actuators regenerate motive energy by storing kinetic energy into an accumulator provided on a hydraulic circuit, or by converting an oil pressure into electricity, whereas electric actuators regenerate electrical energy directly from the kinetic energy obtained during braking. Electric actuators are therefore superior to hydraulic actuators in terms of energy utilization efficiency.
The conventional technique presented in Patent Document 1, for example, employs a hydraulic excavator including an electric motor mounted thereupon as an actuator to drive a swing body. The actuator that swings an upper structure of the hydraulic excavator over a lower structure is used very frequently during work and frequently repeats acceleration and deceleration.
For example, during soil excavation for loading onto a dump truck, after a bucket has been filled with the excavated soil, the hydraulic excavator first turns around and accelerates towards the dump truck. Next in front of the dump truck, the excavator turns around once again, decelerates, and dumps the soil onto a rear body of the dump truck directly from above. After this, the excavator turns around one more time and accelerates towards the location of excavation. Next in front of the location of excavation, the excavator turns around, decelerates, and stops there for further excavation. These steps are repeated.
During this work, if no regenerative operation occurs in hydraulic fashion, then in a case of a hydraulic motor, kinetic energy of the swing body having a heavy inertial load under decelerating conditions, or braking conditions, is given away as heat from the hydraulic circuit since a hydraulic fluid returns to a tank in accordance with a pressure setting of a relief valve.
In a case of an electric motor, on the other hand, since the swing body having the heavy inertial load causes the electric motor to function as an electric power generator, output energy from the electric motor can be regenerated as electrical energy. It is considered from this fact that in terms of energy saving, it is effective to use an electric motor instead of a hydraulic motor.
Using an electric motor to swing an upper structure of a construction machine, however, poses the following problems due to characteristics of the electric motor.
First, maintaining the swing body in a stopped state using the electric motor requires conducting speed feedback control based upon a control variable determined from a comparison between an actual speed and a target control speed. Speed feedback control, however, easily causes hunting due to impacts of a time delay. In addition, during electric motor driving, since an operational feeling is determined by control, particular control performance may cause a feeling of operational discomfort or uneasiness. Furthermore, electric motor or inverter overheating may occur when the electric motor is not rotating and torque is continuously output, for example during ditching when a boom, an arm, and a bucket are shaken/swung to excavate a ditch with the swing body being actuated in jog mode and a side face of the bucket being kept pressed against an inner side face of the ditch. Moreover, use of an electric motor guaranteed to develop an output equivalent to that of a hydraulic motor may pose a problem of motor oversizing or a significant increase in cost.
In order to solve the above problems, Patent Documents 2 and 3 disclose construction machines adapted to realize energy saving and to include both of a hydraulic motor and an electric motor and drive or brake a swing body by use of a total torque of the motors.
The conventional technique disclosed in Patent Document 2 employs an energy-regenerating device of a hydraulic construction machine in which an electric motor for swinging is directly coupled to a hydraulic motor for swinging and a controller sends an output torque command to the electric motor in accordance with the amount of manipulation of a control lever for swinging. During deceleration, that is, during braking, the electric motor in the conventional technique regenerates kinetic energy of the swing body and stores the energy into a battery as electrical energy.
The conventional technique disclosed in Patent Document 3 employs a hybrid-type construction machine that uses a differential pressure between a meter-in circuit and meter-out circuit of a hydraulic motor to calculate a torque command value to be assigned to an electric motor, for adequate output torque allocations between the hydraulic motor and the electric motor.
Both of the conventional techniques disclosed in Patent Documents 2 and 3 use a hydraulic motor and an electric motor in combination as actuators for swinging. Both techniques, therefore, ensure a sufficient torque necessary to drive the swing body, and recover electrical energy with the electric motor. Both also save energy by adopting a simplified and easy-to-commercialize system configuration as the swing body driving system in the construction machine.